In an industrial process, it is often desirable to maintain the value of a process parameter, referred to as the controlled variable, at a particular setpoint value. The controlled variable is typically controlled by adjusting the value of another process parameter referred to as a manipulated variable. For example, in a nuclear power plant, one controlled variable is the temperature of the circulating coolant, and one manipulated variable is the extent to which neutron absorbing rods penetrate the reactor core to slow down the nuclear reaction responsible for raising the temperature.
The task of correctly adjusting the value of a manipulated variable, in order to control the value of a controlled variable, most often falls on a feedback control system associated with the industrial process. In a feedback control system, a sensor measures the value of the controlled variable and provides that value to a controller. On the basis of the difference between the measured value of the controlled variable and the setpoint, the controller determines the value of the manipulated variable required to bring to zero the difference between the controlled variable and the setpoint.
Once the controller determines the desired value of the manipulated variable, it transmits a control signal to drive the value of the manipulated variable towards that desired value. Almost invariably, this control signal results in the operation of a switch associated with a process actuator. For example, in the case of the nuclear power plant, the controller may transmit a control signal that closes a relay switch. This relay switch then connects a power supply to a load, such as an electric motor that moves the neutron absorbing rods into the reactor core in order to slow the reaction.
In many cases, the control signal does not close the switch directly. This is because controllers are typically low power digital devices primarily intended for information processing and not for providing the power required to operate large relay switches. In these cases, the controller transmits a control signal to an output module whose function is to connect the relay switch to a power supply having sufficient power to drive the relay switch. An output module can thus be considered as an actuator for the actuator.
It is possible, of course, for a controller to operate flawlessly, but for an operating component within either the output module or the actuator to fail. For example, a switch within the output module may fail to open or close in response to the control signal. Alternatively, the power supply for driving the load may fail. Either of these failures will eventually be manifested by measured values of the controlled variable that are grossly inconsistent with the control signal. This inconsistency can prompt the controller to generate an alarm. The disadvantage of detecting a failure in this manner is that there is often such a lengthy delay before the seriousness of the situation becomes apparent that by the time the problem is known, it is already too late to do anything about it.
It is known to provide a self-validating output module in which a series resistance is placed between a load, for example a relay switch, and a power supply. The voltage across the series resistor can then be measured by a differential amplifier, the output of which is made available to the controller. The presence or absence of a voltage across the series resistor in this type of output module indicates whether or not the switch connecting the load to the power supply is open or closed. If the switch is closed, a voltage drop proportional to the drawn current will exist across the series resistor. Conversely, if the switch is open, no voltage drop (or a negligible voltage drop due to leakage current, in the case of a transistor switch) will exist across the series resistor.
A disadvantage of the foregoing self-validating output module is the additional cost and complexity associated with providing a differential amplifier with its own power supply. A more serious disadvantage is that this type of self-validating output module cannot readily verify that the power supply is ready and able to provide the necessary power to the load. This is because a lack of voltage across the series resistor is consistent with both an open switch in conjunction with a working power supply, and an open switch in conjunction with a malfunctioning power supply. It is only when the switch is closed that one can determine, on the basis of the voltage across the series resistor, whether the power supply can deliver.
What is necessary in the art therefore is a self-validating output module having simplicity of construction and the ability to identify a malfunctioning power supply.
The invention provides a self-validating output module that includes an operating component that operates in either a first state or a second state. When operating in the first state, the operating component directs current towards a current detector and away from a load. Conversely, when operating in the second state, the operating component directs current away from the current detector and towards the load. This self-validating output module further includes a signal generator responsive current in the current detector. In response to the presence or absence of current in the current detector, this signal generator generates a status signal indicative of whether the operating component is operating in the first state or in the second state. This status signal can be available to a controller. If the status signal indicates a malfunction, the controller can then immediately alert a human operator.
In one embodiment of a self-validating output module according to the invention, the operating component is a switch having a first state in which it directs current from a power supply to a load and having a second state in which it directs current from the power supply to the current detector. Typically, the load is a relay switch for changing a manipulated variable of a controlled process. The load can be either within the output module itself, but is most commonly associated with a process actuator outside the output module.
In a preferred embodiment, the current detector is a unidirectional current carrier such as a diode. The output of the diode is electrically coupled to the control terminal of a transistor so that the presence or absence of current in the current detector results in the presence or absence of a conducting path between two other terminals of the transistor.
If electrical isolation of the output module is desired, the diode can be a light-emitting diode optically coupled to the control terminal of a transistor operating in the manner described above. The combination of a light-emitting diode optically coupled to a transistor is known in the art as an optoisolator and is frequently used in applications in which electrical isolation is desired between a sensor and a sensed parameter.
If communication of the status signal across a network is desirable, the output module can further include a processor in communication with a network. In this embodiment, the processor executes instructions for transforming the output of a logic circuit into a message suitable for transmission on a network.
The operating component can also be a power supply connected to a load. In such an embodiment, the power supply has a first state in which it supplies sufficient power to drive the load, and has a second state in which it fails to provide sufficient power to drive the load. In this embodiment, a self-validating output module according to the invention enables the controller to verify that the power supply is ready and able to supply power to the load. This ability to determine the output of the power supply, without actually connecting the power supply to the load, can be further enhanced by providing a voltage detector configured to detect voltage in excess of a threshold required to operate the load. Such a voltage detector can be implemented as a zener diode in series with a current detector and having a breakdown voltage selected to permit reverse conduction through the zener diode when the power supply generates a voltage in excess of the desired threshold.
The invention also includes a method of transmitting a status signal to a process controller that indicates the status of an operating component within an output module. The method includes the step of directing current to a current detector and away from a load when the operating component is in a first state and directing current and directing the current to the load and away from the current detector when the operating component is in the second state. The presence or absence of current in the current detector is thus indicative of the whether the operating component is in the first state of the second state. The method further includes the step of generating a status signal on the basis of whether there exists current in the current detector.
These and other features of the invention will be further apparent in connection with the following detailed description and the accompanying figures, in which: